1. Field of the Invention
The present invention relates to a semiconductor laser device and more particularly to a semiconductor laser device comprising an InGaAsP active layer with compressive strain on a GaAs substrate.
2. Description of the Related Art
In “High-Power Highly-reliable Operation of 0.98-μm InGaAs—InGaP Strain-Compressed Single-Quantum-Well Lasers with Tensile-Strained InGaAsP Barriers.: IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2 (1995) pp. 189.” (hereinafter referred to as document 1), as an all Al-free semiconductor laser device with a wavelength of 0.98 μm, there is reported a semiconductor laser device. This semiconductor laser device is provided with a current confinement layer and a refractive index waveguide mechanism and which oscillates in a fundamental transverse mode, wherein on an n-type GaAs substrate, laminated are an n-type InGaP cladding layer, an InGaAsP optical waveguide layer of an undoped type, a tensile-stained InGaAsP barrier layer, an InGaAs double quantum well active layer, a tensile-stained InGaAsP barrier layer, an InGaAsP optical waveguide layer of an undoped type, a p-type InGaP upper first cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP upper second cladding layer, a p-type GaAs cap layer and an insulation layer so as to form a narrow-striped ridge structure up to the upper portion of the p-type InGaP upper first cladding layer by selective etching with general photolithography, and both sides of the ridge structure are buried with n-type In0.5Ga0.5P by selective MOCVD growth to remove the insulation layer so as to form a p-type GaAs contact layer.
However, in the semiconductor laser device reported in document 1, a characteristic temperature of a threshold current is as low as 156 K, causing difficulty in reducing costs since temperature adjustment is essential to practical use. The reason why the temperature characteristic of this semiconductor layer device is low is largely attributed to influence due to carrier overflow. The carrier overflow is a phenomenon in which density of carrier injection to the active layer is increased by high power and a high-temperature operation, so that the carrier overflows from the active layer area onto the optical waveguide layer and cladding layer area. Particularly, in the case of document 1, there has been a drawback in which the structure causes the carrier overflow easily since a band gap difference between the active layer and the optical waveguide layer is small.
In order to solve this problem, there has been proposed a semiconductor laser device using In0.49Ga0.51P as an optical waveguide layer and an Alz1Ga1−z1As cladding layer (0.6≦z1≦0.8) as a cladding layer in Japanese Unexamined Patent Publication No. 2001-168458. The adoption of In0.49Ga0.51P with a large band gap as an optical waveguide layer suppresses the leakage of carriers from the active layer and a value of a characteristic temperature T0 reaches 300 K or more. Therefore, a large improvement in the temperature characteristic can be seen. Moreover, in the aforementioned publication, as an example that is applied to a single mode LD, there is disclosed the structure in which In0.49(Alz2Ga1−z2)0.51P (0.15≦z2≦1) is used as a current confinement layer and this makes it possible to obtain a single mode LD with a good temperature characteristic. However, in In0.49(Alz2Ga1−z2)0.51P with high Al composition, there were problems such as deterioration in characteristics, a reduction in yields, etc., resulting from the fact that a good quality crystal was not easily obtained. Accordingly, the aforementioned semiconductor laser device still had room for improvement.
Meanwhile, in Japanese Unexamined Patent Publication No. 2001-148541, in order to improve noise characteristics of a semiconductor laser device, there is proposed a structure wherein in the semiconductor laser device using Inx1Ga1−x1As1−y1Py1 as a quantum well active layer and In0.49Ga0.51P as an optical waveguide layer, an In0.49Ga0.51P upper first optical waveguide layer, a GaAs etching stop layer, an In0.49(Alz2Ga1−z2)0.51P current confinement layer, and a In0.49Ga0.51P cap layer are laminated as a layer structure on the quantum well active layer, thereafter the In0.49Ga0.51P cap layer of a stripe region where a current is injected, the In0.49(Alz2Ga1−z2)0.51P current confinement layer, and the GaAs etching stop layer are removed to expose the In0.49Ga0.51P upper first optical waveguide layer, and two layers of an In0.49Ga0.51P upper second optical waveguide layer and an Alz1Ga1−z1As upper cladding layer (0.57≦z1≦0.8) are laminated on the cap layer and the upper first optical waveguide layer.
The semiconductor laser device having the aforementioned structure has an advantage in that not only a noise characteristic is improved but also the Al composition of In0.49(Alz2Ga1−z2)0.51P, which is the current confinement layer, can be reduced as compared with the semiconductor laser device disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2001-168458. However, there was still more room for improvement in the suppression of catastrophic optical damage (COD) under a high power drive condition even with the exception of the problem resulting from the high Al composition.